Whether it’s from Donald Trump or Bernie Sanders, the anti-trade movement in the US presidential election would boost China’s prospects, not America’s.

Free trade has become a major topic in the US presidential election. Both Republicans and Democrats are campaigning against free trade. If the US turns its back on trade, though, it won’t be American workers who will benefit. It may be China’s.

Donald Trump, the leading contender for the Republican ticket, has threatened to erect a 35% tariff barrier against imports from China and Japan. Bernie Sanders, the self-proclaimed socialist competing for the Democratic nomination,...